Several different and incompatible encryption systems are currently in use in cable television systems. In general, each encryption system is specific to a particular manufacturer and is maintained as a proprietary system. When a cable system operator (or other content distributor) builds a system around a particular manufacturer, it becomes difficult and expensive to change to another manufacturer that may provide lower cost or higher performance hardware. Thus, a content distributor is often locked into a single source of hardware (e.g., television set-top boxes).
This problem can be avoided somewhat by using a technique known as “dual carriage” (or “multiple carriage”) of encrypted content. In this technique, the same program is duplicated with each copy sent with a different type of encryption. Thus, multiple set-top boxes from multiple manufacturers can coexist on the same system. Unfortunately, this technique has a serious bandwidth penalty due to the need to transmit duplicate copies of all content.
The above-referenced patent applications describe techniques referred to as “partial encryption” or “selective encryption”. These techniques are used to effectively permit a virtual form of “dual carriage” (or multiple carriage) of a television program over a single distribution system (e.g., a cable television system) using multiple encryption techniques. By only partially encrypting a particular program (i.e., only encrypting certain portions of the digital data associated with a program), multiple copies of the encrypted portion of the program can be carried over the distribution system with the remaining content carried in the clear. These techniques permit a virtual form of dual carriage (or multiple carriage) of the program content with a minimal bandwidth penalty. A significant advantage of such a system is that the content provider (e.g., a cable television system operator) can use television set-top boxes (STBs) provided by multiple manufacturers that encrypt content under multiple encryption systems without suffering a large bandwidth penalty.
In a conventional cable system, system information (SI) is provided in the form illustrated in FIG. 1 of a Program Association Table (PAT) which contains an entry for each program. Each program in the PAT has a pointer to a particular Program Map Table (PMT) such as 12, 14, . . . 18 and 20 associated with the particular program. The PMT table contains packet Identifiers (PIDs) that are associated with the elementary streams for each program.
In the above-referenced patent applications, the multiple sets of encrypted packets representing the encrypted portions of the partially encrypted programs are distinguished from one another by use of distinctive packet identifiers (PLDs). Thus, for example, two encrypted portions of a program have two unique PIDs—a primary PID and a shadow (or secondary) PID. In order for the receiving equipment to determine which PIDs are associated with a particular encryption scheme, the PIT) information is transmitted from the cable system (or other distributor) headend. In one embodiment, illustrated in FIG. 2, this can be done using a duplicate set of system information (SI) to identify the various PIDs. In this example, two separate PATs 30a and 30b are used to associate programs with PMTs 32a, 34a, . . . 38a and 40a in the case of PAT 30a, and with 32b, 34b, . . . 38b and 40b in the case of PAT 30b. Each receiving system is able to detect and process whichever SI is appropriate. The system application Ser. No. 10/084,106 (e.g., the cable system headend) generating the SI creates duplicate SI for each encryption scheme used. When bandwidth is critical, the extra packets used to transmit the duplicate SI may be difficult to accommodate.
Systems that are aware that shadow PIDs exist need to know of the PID pairs and reconstitute the merged stream. The system then needs a method to reconstitute the shadow stream from the payloads of both PIDs.